32 Thermodynamics of the Exchange of
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Alkylammonium Ions in Synthetic Faujasites E. F. VANSANT and J. B. UYTTERHOEVEN University of Leuven, de Croylaan 42, 3030 Heverlee, Belgium
The thermodynamics of exchange of alkylammonium ions in synthetic faujasites is studied. A correlation exists be tween the maximum limit to exchange and the selectivity. Both properties are controlled by the same factors. An adapted form of the Gaines and Thomas formula is pro posed, allowing the calculation of the equilibrium constant when the exchange reaction is incomplete. Values of ΔG, ΔΗ, and ΔS were calculated. Interaction with the lattice is the most important factor in the exchange with low alumi num faujasites (Y). In the X-like samples, exchange seems to be ruled by the change in hydration state of the large cavities. A possible redistribution of the exchangeable cat ions during the exchange reaction is assumed.
T o n sieve effects are o b s e r v e d i n i o n exchange reactions o n synthetic faujasites w h e n t h e d i m e n s i o n s of t h e i n g o i n g i o n are s u c h t h a t t h e y cannot diffuse i n s i d e t h e s m a l l cavities. T h e s e p h e n o m e n a w e r e r e p o r t e d b y S h e r r y (8, 9) for exchange w i t h i n o r g a n i c ions a n d b y B a r r e r a n d c o w o r k e r s ( J ) f o r t h e exchange of o r g a n i c ions. B a r r e r e m p h a s i z e d t h e influence of space r e q u i r e m e n t s , w h i c h h e c o n s i d e r e d to b e t h e m a i n factor d e t e r m i n i n g t h e m a x i m u m l i m i t to exchange. T h e n g et al. (12)
s t u d i e d t h e exchange of several a l k y l a m m o n i u m
ions o n a t y p i c a l X a n d a t y p i c a l Y zeolite. T h e s e ions c o u l d o n l y exchange i n t h e large cavities. T h e y c o n c l u d e d that the exchange is l o w e r t h a n the l i m i t i m p o s e d b y possible space r e q u i r e m e n t s . T h e h m i t t o exchange w a s different f o r t h e X a n d Y zeolites a l t h o u g h t h e a v a i l a b l e v o l u m e is almost t h e same. T h e m a x i m u m l i m i t to exchange w a s a l i n e a r f u n c t i o n of t h e m o l e c u l a r w e i g h t of t h e i n g o i n g ions. T h e decrease of that f u n c t i o n w i t h i n c r e a s 426 In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
32.
VANSANT AND UYTTERHOEVEN
Alkylammonium
427
Ions
i n g m o l e c u l a r w e i g h t w a s m o r e p r o n o u n c e d i n X t h a n i n Y . T h e exchange l i m i t w a s i n f l u e n c e d also b y the n a t u r e of the ions i n i t i a l l y present. F r o m the c o r r e l a t i o n b e t w e e n the l i m i t to exchange a n d the AG of T h e n g et al. (12)
exchange,
c o n c l u d e d that the m a x i m u m h m i t to exchange
was
d e t e r m i n e d b y the same factors as the exchange selectivity. T h e y assume that the d i s t r i b u t i o n of the ions over different exchange sites i n the s m a l l a n d large cavities is not fixed b u t c o n t r o l l e d b y the n a t u r e of t h e e x c h a n g i n g ions. T h e s e hypotheses w e r e c o n f i r m e d b y V a n s a n t et ah (13)
in a
s t u d y of the exchange of a r o m a t i c a n d h e t e r o c y c l i c a m m o n i u m d e r i v a Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 1, 2015 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch032
tives. A v e r y p r o n o u n c e d s e l e c t i v i t y i n f a v o r of these o r g a n i c ions w a s o b s e r v e d a n d , i n agreement w i t h the hypothesis of T h e n g et ah (12),
the
a m o u n t of i n g o i n g ions w a s also v e r y h i g h . P a c k i n g c a l c u l a t i o n s p r o v e d a m a x i m u m filling of t h e z e o l i t e cages w i t h the a r o m a t i c a n d h e t e r o c y c l i c a m m o n i u m ions. I n this w o r k , a s t u d y is m a d e of the exchange of the n - a l k y l a m m o n i u m ions, g o i n g f r o m N H
4
to b u t y l a m m o n i u m , o n 4 s y n t h e t i c faujasites
+
d i f f e r i n g b y t h e i r A l content.
A n extensive t h e r m o d y n a m i c treatment of
the exchange w i t h the p r o p y l a m m o n i u m i o n is i n c l u d e d . Experimental Materials.
F o u r different samples w i t h A l / S i ratios r a n g i n g f r o m
a n extreme Y to a n extreme X w e r e s u p p l i e d b y the L i n d e C o . , N e w Y o r k . A s t h e y constitute a c o m p l e t e s u b s t i t u t i o n a l series, the i n d i c a t i o n s X a n d Y w i l l not b e u s e d f u r t h e r b u t w i l l b e r e p l a c e d b y the s y m b o l F f o l l o w e d b y the n a m e of the s a t u r a t i n g i o n a n d a figure i n d i c a t i n g the n u m b e r of ions p e r u n i t c e l l . I n o r d e r to ensure c o m p l e t e s a t u r a t i o n , the samples as r e c e i v e d w e r e t r e a t e d w i t h a I N N a C l s o l u t i o n a n d w a s h e d free excess electrolyte.
from
T h e c h e m i c a l c o m p o s i t i o n w a s d e t e r m i n e d a n d ex-
pressed i n terms of u n i t c e l l f o r m u l a , as f o l l o w s : Composition
Sample F F F F
N a Na Na Na
49 55 71 85
Na (A10 ) Na (A10 ) Na , (AlOOn N (A10 ) 4 9
6 5
2
4 9
2
B5
2
86
7
a 8 5
( S i 0 ) 143 ( S i O i ) 137 ( S i O i ) 121 ( S i O i ) 107 2
T h e symbols M e , E t , P r , a n d B u w i l l b e u s e d to s y m b o l i z e the ions methylammonium, ethylammonium, propylammonium, and butylammon i u m , respectively. Exchange Procedure. Zeolite samples of 0.1 g r a m w e r e e q u i l i b r a t e d o v e r n i g h t i n 40 m l of a s o l u t i o n c o n t a i n i n g a p p r o p r i a t e p r o p o r t i o n s of N a a n d a l k y l a m m o n i u m ions.
A fixed t e m p e r a t u r e
( 4 ° , 17°, 25°, 40°,
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
+
or
428
MOLECULAR SIEVE ZEOLITES
5 5 ° C ) w a s m a i n t a i n e d d u r i n g the e q u i l i b r a t i o n . T h e a m o u n t of
1
alkyl-
a m m o n i u m ions a d s o r b e d w a s c a l c u l a t e d f r o m the v a r i a t i o n i n the c o n c e n t r a t i o n of t h e e q u i h b r i u m s o l u t i o n . T h e v a l i d i t y of this m e t h o d tested i n a f e w cases b y m a k i n g a c o m p l e t e s t o i c h i o m e t r i c b a l a n c e . details of the a n a l y t i c a l p r o c e d u r e s h a v e b e e n g i v e n elsewhere
was More
(II).
T h e results of the exchange c a n b e expressed as e x c h a n g e isotherms p l o t t i n g the e q u i v a l e n t f r a c t i o n of the exchange c a p a c i t y of the zeolite (Z)
saturated w i t h a l k y l a m m o n i u m as a f u n c t i o n of S, the
f r a c t i o n of a l k y l a m m o n i u m ions i n the solution.
equivalent
T h e isotherms for the
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p r o p y l a m m o n i u m exchange are g i v e n i n F i g u r e 1. T h e m a x i m u m l i m i t s to exchange w e r e d e t e r m i n e d o n separate samples.
These were equili-
b r a t e d w i t h 0 . 5 N solutions of the a p p r o p r i a t e a l k y l a m m o n i u m chlorides. T h e equilibrated solution was renewed
t w i c e ; t h e n the samples
w a s h e d free f r o m excess a l k y l a m m o n i u m , a n d the a d s o r b e d o r g a n i c ions w a s a n a l y z e d .
Figure 1.
were
amount
of
T h e m a x i m u m l i m i t s to e x c h a n g e o b t a i n e d
Exchange isotherms of the propylammonium samples at different temperatures
ions in the
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
different
32.
Alkylammonium
VANSANT A N D U Y T T E R H O E V E N
429
Ions
i n t h i s w a y c o r r e s p o n d e d to t h e l i m i t s t h a t c o u l d b e d e r i v e d f r o m t h e isotherms u s i n g t h e e x t r a p o l a t i o n p r o c e d u r e
proposed
b y Barrer a n d
M e i e r ( 3 ) . T h i s p r o c e d u r e is b a s e d o n t h e l i n e a r r e l a t i o n s h i p b e t w e e n Z
M
and
^ p
Z.
1
M
Estimation of Thermodynamic Functions for Partial Exchange. A n exchange of ions b e t w e e n a s o l i d a n d a l i q u i d phase c a n b e r e p r e s e n t e d by: N a
+
( 2 )
+
M
+
(
s
)
^
+
M\
z)
Na
+
( s )
.
T h e subscripts ζ a n d s refer to
the z e o l i t e a n d t h e l i q u i d phase. A c c o r d i n g to G a i n e s a n d T h o m a s
(6),
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the t h e r m o d y n a m i c e q u i l i b r i u m constant is expressed as f o l l o w s :
InK
=
J
i.
In K d c
w h e r e K is the " c o r r e c t e d " s e l e c t i v i t y coefficient d e f i n e d as c
__ Z S^ M
^ y Ν a,
a
Ζ a n d S are the e q u i v a l e n t fractions of t h e ions o n t h e z e o l i t e a n d i n s o l u t i o n , a n d γ is the a c t i v i t y coefficient of the ions i n s o l u t i o n . W i t h a t o t a l i o n c o n c e n t r a t i o n of 0 . 0 5 N , t h e r a t i o of t h e a c t i v i t y coefficients does n o t d e v i a t e s i g n i f i c a n t l y f r o m u n i t y ( 7 ). T h i s expression of t h e e q u i l i b r i u m constant c a n b e u s e d o n l y i f there is a exchange.
complete
A n u m b e r of w o r k e r s a p p l i e d this e q u a t i o n o n i n c o m p l e t e
exchanges i n zeolites
after a n o r m a l i z a t i o n p u t t i n g t h e e x p e r i
(8-12)
m e n t a l m a x i m u m exchange e q u a l to u n i t y ( Z
max
M
=
1) ·
S u c h a n o r m a l i z a t i o n assumes that the r e s i d u a l N a ions are i n a c +
cessible f o r exchange a n d h a v e n o influence o n t h e s e l e c t i v i t y coefficient. T h a t this a s s u m p t i o n is i n c o r r e c t results f r o m t h e fact t h a t t h e m a x i m u m l i m i t to exchange is i n f l u e n c e d b y t h e n a t u r e of t h e o u t g o i n g a n d i n g o i n g ions, a n d also b y t h e A l / S i ratios of t h e zeolites.
I n o r d e r to o b t a i n
c o m p a r a b l e values of the e q u i l i b r i u m constant, w e m o d i f i e d t h e G a i n e s and Thomas equation.
T h e c o r r e c t e d a n d n o r m a l i z e d s e l e c t i v i t y coeffi
cient is d e f i n e d as
g
N
_
ZMJ (1 —
where / =
l/Z
.
m&x
M
$ N a
ZM/)SM
I n t r o d u c t i o n of this K / i n t h e G a i n e s a n d T h o m a s
e q u a t i o n r e q u i r e s a c o r r e c t i o n t e r m t a k i n g i n t o a c c o u n t t h e influence of the r e s i d u a l N a ions.
T h e Gaines a n d Thomas equation n o w can be
w r i t t e n as 1
In Κ =
I
f i n K»\
J J
~ l {d M
J — AM]
Zf M
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
430
MOLECULAR SIEVE ZEOLITES
Κ represents t h e r e a l e q u i l i b r i u m constant f o r a n exchange Ζ =
0 and Ζ =
as AG =
Z
m a x
.
1
between
T h e s t a n d a r d free energy of exchange is o b t a i n e d
—RT In K. T h e e n t h a l p y of exchange is d e r i v e d f r o m t h e V a n t
H o f f e q u a t i o n , a n d the e n t r o p y change is c a l c u l a t e d f r o m the AG a n d Δ Η values: IAG/T 8
Γ
AH =
ψϊ
^ a
n
c
x
r
. „
i
=
*
AH
_ —
4
σ
TAS
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Results T h e exchange isotherms, established w i t h the i o n P r o n t h e 4 zeolite samples, are g i v e n i n F i g u r e 1. T h e exchange w a s i n d e p e n d e n t of t h e t e m p e r a t u r e o n the samples w i t h h i g h a l u m i n u m content:
FNa85 and
F N a 7 1 . O n the other samples, e s p e c i a l l y F N a 5 5 , t h e extent o f
exchange
increases f r o m 4 ° to 25 ° C , i n d i c a t i n g a n e n d o t h e r m i c process, a n d d e creases a g a i n at h i g h e r temperatures, w h i c h is t y p i c a l f o r a n e x o t h e r m i c process.
T h e values of AG, AH, a n d AS d e r i v e d f r o m these isotherms are
c o l l e c t e d i n T a b l e I. T h e o v e r - a l l order of affinity of the zeolites f o r the p r o p y l a m m o n i u m i o n , d e r i v e d f r o m AG values i n T a b l e I , is : F N a 85
